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Cambridge, MA-based Ironwood Pharmaceuticals has some new evidence out this morning that its lead drug candidate is an effective treatment for patients with chronic constipation.

Ironwood and its partner, New York-based Forest Laboratories (NYSE: FRX) said last night that the drug, linaclotide, passed the test in a pair of pivotal clinical trials. A significant percentange of patients who got a once-daily dose of the drug experienced relief from their constipation, the primary goal of the studies, as well as relief from bloating and abdominal discomfort. The most common adverse events in patients on the Ironwood drug were diarrhea, flatulence and abdominal pain; 7.4 percent of patients taking the drug dropped out of the studies, compared to 4.2 of the patients who were on a placebo.

This result means that Ironwood and Forest essentially have two down, two to go with their clinical-trial plan for the new drug. About 1,270 patients were enrolled in these two constipation trials, and the companies expect results from two more pivotal trials among 1,600 more patients who have irritable bowel syndrome with constipation. Results from those trials are expected in the second half of 2010, the companies said. Forest has been quite bullish on this drug’s prospects, noting that estimated 26 million Americans have chronic constipation, there are few available therapies for it, and the Ironwood drug has a novel mode of action. The U.S. health care system spends more than $25 billion a year on treating irritable bowel syndrome, according to Forest Laboratories’ annual report. If approved, linaclotide could generated $572 million in annual sales by 2015, according to Tom Russo, an analyst with Robert W. Baird.

The results “confirm the potential for linaclotide to bring relief to the millions of patients suffering from many of the symptoms associated with chronic constipation,” said Forest CEO Howard Solomon in a statement. Ironwood CEO Peter Hecht noted that the company was glad to see the pivotal study results reproduce what was observed in smaller studies.

Ironwood has been able to raise more than $306 million since its founding, and accumulate more than $100 million in the bank now, thanks largely to having a drug with an unusual profile targeted at a common condition that’s tough to treat. Linaclotide is a peptide, and while most peptides are injected, Ironwood turned its drug into an oral pill that can withstand stomach acids, work its way into the intestines, and do its job there, without being absorbed into the bloodstream, where it can cause side effects, Hecht has said. The drug is thought to act by stimulating secretions of fluids into the intestines, which softens stool and helps people have easier bowel movements.

Digging a little further into the data, this drug doesn’t appear to help everybody. Both studies considered a patient a “responder” if he or she reported at least three complete, spontaneous bowel movements per week, and an increase of at least one of those bowel movements per week over a baseline period for at least nine weeks during the 12-week treatment period.

In the first study, about 16 percent of patients on a low dose of linaclotide achieved that definition of success, while 21.3 percent did that well on a higher dose, compared with about 6 percent of patients taking a placebo, the companies said. The second study found the low dose was actually slightly better than the high dose (21.2 versus 19.4 percent response rates), compared to 3.3 percent in the placebo group.

While that might not sound like an amazing response rate for the drug, these patients sound they were feeling pretty bad entering the study. During that two-week baseline period of the study, 72 percent of the patients had no complete, spontaneous bowel movements. The medical term is defined as when a patient reports they felt a complete emptying of the bowel.